How do dirt roads get the washboard surface?

Answer 1

Any bed of dirt or sand, even a very smooth one, has minuscule irregularities that slightly jog a rolling wheel. Each time the wheel hits a bump, it pushes the dirt forward a bit, enlarging the irregularity. Then, as the wheel passes over the top of the bump, the force of its descent pushes dirt forward into the next bump. Repeat these actions a hundred or more times and the familiar pattern of ridges appears.